In conventional fiber optic transceiver modules, it is common to use a photodetector to monitor the output of the transmitting laser. Usually, the transmitting laser beam is partially reflected back onto the photodetector by way of a semi-transmissive mirror (also known as a semi-reflective window.) The mirror is placed between the laser and the focusing lens such that the light from the laser hits the mirror first, thus reflecting a portion of the light onto the photodetector. This reflected light is sensed by the photodetector and used to provide feedback to the laser driver. In this manner, it is possible to monitor the power output of the laser and adjust it to compensate for variations in laser output power due to, for example, temperature changes in the environment in which the laser is operating. The light that is not reflected travels through the mirror and strikes the lens, where it is focused on an optical fiber. This type of construction is especially true for most fiber optic transceivers using surface emitting lasers such as VCSELs.
There are currently several known configurations used by fiber optic transceiver module manufacturers in placing the mirror relative to the optical axis (i.e. in the direction of the propagated light) of the module. In a first known configuration, the surface of the semi-transmissive mirror is located between the laser and the lens such that light strikes the mirror normally (i.e. perpendicular to the optical axis.) Light reflecting off a mirror in this configuration will be directed back towards the laser source. Consequently, the monitor photodetector must normally be placed under the laser. Also, in this configuration, reflected light will strike the laser source as well as the monitor photodetector. This light from the laser reflecting back to the same laser is normally undesirable and may cause increased noise (normally referred to as Relative Intensity Noise or RIN for short) in the transmission signals. Another disadvantage is that since part of the reflected light will strike the laser and that the laser is normally placed on top of the photodetector, not all the reflected light gets to the monitor photodetector (leading to reduced efficiency).
In a second known configuration, the semi-transmissive mirror may be placed at an angle to the optical axis. In this configuration, light reflecting off the mirror will be directed away from the laser source and onto the monitor photodetector, which is normally located next to the laser source, and the distance between the two depends on the angle of the mirror. One disadvantage of this configuration is that the monitor photodetector must be made larger in order to capture all the reflected light. However, this is normally difficult to achieve.
A disadvantage of conventional designs of fiber optic transmitter optical sub-assembly (or T-OSA) is in the complexity of the mirror and lens assembly, e.g., normally, the mirror and the lens must be positioned separately. Another disadvantage is in the relatively large size of current assembly modules, due in part to the need for a relatively large monitor photodetector.
Therefore, an integrated focusing lens and reflector that reduces the number of process steps, improves assembly tolerance, is more tolerant to temperature change, better controls and focuses reflected light to a monitor photodetector, and provides a significantly smaller lens/reflector system, is needed.